Postgresql – How to delete records without a primary key in a stardard way (i.e. not using ctid)

deleteduplicationpostgresql

Case of use

We have a log table without a PK with two columns (date, event);

You inserted a row. Then you want to undo the inserted record.

Is there a way to delete that row (without deleting other rows with the same data), that doesn't use postgres only capabilities?

I want a solution that works in another database (it doesn't need to be totally standard, it can be only in just one database: SqLite, Oracle, MySql or SQLServer).

Example:

create table the_log(
  date date,
  event_id integer
);

insert into the_log(date, event_id) values ('2019-09-21',1),('2019-09-21',1);

select * from the_log;

My atempts:

delete from the_log where row_number() over ()=1;

delete from the_log limit 1;

with the_log_2 as (select *, row_number() over () as version from prueba_sin_clave)
delete from the_log_2 where version=1;

I supouse that the answer is No. I want to know if I am wrong or in what documentation I read that I am right.

Best Answer

CAUTION : This can make you lose data, so consider using transaction block.

I think this is the more generic solution:

If you consider that the line you want to delete is:

select * from the_log limit 1

then if you have one column that has a unique value...

delete from the_log  
where unique_the_log_column in 
(select unique_log_column from the_log limit 1);

If you do not have a unique column, but you have two columns that combined generates a unique value:

delete from the_log  
where (col1, col2) in (select col1, col2 from the_log limit 1);

Where any_the_log_column is a column with a unique value in this table.

This will probably delete the first line of the log table in any dbms - if data in column is not repeated, but remember that tables sometimes is not selected ordered in some databases so you must have a id in this table...

Consider creating a id column and increasing it by the numbers of the lines in the table, based on date or something if you don't want to get hurt in the future...

Related Solutions

DB2 – Remove All Records with Duplicates

Assuming your desired uniqueness constraint is (one,two), here is one way to do it:

DELETE FROM session.test WHERE (one, two) IN ( SELECT one, two FROM session.test GROUP BY one, two HAVING COUNT(*) > 1 )

PostgreSQL – How to Delete Duplicate Records Efficiently

Core feature is the window function lag().
Also pay special attention to avoid deadlocks and race conditions with concurrent deletes and inserts (which can affect which rows to delete!):

CREATE OR REPLACE FUNCTION remove_vendor_price_dupes(_vendor int)
  RETURNS integer AS
$func$
DECLARE
   del_ct int;
BEGIN
   -- this may or may not be necessary:
   -- lock rows to avoid race conditions with concurrent deletes
   PERFORM 1
   FROM   vendor_prices
   WHERE  vendor = _vendor
   ORDER  BY sku, effective_date, id  -- guarantee row locks in consistent order
   FOR    UPDATE;

   -- delete redundant prices
   DELETE FROM vendor_prices v
   USING (
      SELECT id
           , price = lag(price) OVER w  -- same as last row
             AND (lead(id) OVER w) IS NOT NULL AS del  -- not last row
      FROM   vendor_prices
      WHERE  vendor = _vendor
      WINDOW w AS (PARTITION BY sku ORDER BY effective_date, id)
      ) d
   WHERE v.id = d.id
   AND   d.del;

   GET DIAGNOSTICS del_ct = ROW_COUNT;  -- optional:
   RETURN del_ct;  -- return number of deleted rows
END
$func$  LANGUAGE plpgsql;

Call:

SELECT remove_vendor_price_dupes(1);

Notes

The current version of the 9.3 major release is 9.3.6. The project recommends that ...

all users run the latest available minor release for whatever major version is in use.
A multicolumn index on (vendor, sku, effective_date, id) would be perfect for this - in this particular order. But Postgres can combine indexes rather efficiently, too.
It might pay to add the otherwise irrelevant price as last item ot the index to get index-only scans out of this. You'll have to test.
Since you have concurrent deletes it may be a good idea to run a separate delete per vendor to reduce the potential for race conditions and deadlocks. Since there are only a few vendors, this seems like a reasonable partitioning. (Many tiny calls would be comparatively slow.)
I am running a separate SELECT (PERFORM in plpgsql, since we do not use the result) because the row locking clause FOR UPDATE cannot be used together with window functions. Don't let the keyword mislead you, this is not just for updates. I am locking all rows for the given vendor, since the result depends on all rows. Concurrent reads are not impaired, only concurrent writes have to wait until we are done. That's another reason why deleting rows for one vendor at a time in a separate transaction should be best.
sku is unique per product, so we can PARTITION BY it.
ORDER BY effective_date, id: your first version of the question included code for duplicate rows, so I added id to ORDER BY as additional tie breaker. This way it works for duplicates on (sku, effective_date) as well.
To preserve the last row for each set: AND (lead(id) OVER w) IS NOT NULL. Reusing the same window for lead() is cheap - independent of the added explicit WINDOW clause - that's just syntax shorthand for convenience.
I am locking rows in the same order: ORDER BY sku, effective_date, id. Make sure that concurrent DELETEs operate in the same order to avoid deadlocks. If all other transactions delete no more than a single row within the same transaction, there cannot be deadlocks and you don't need the row locking at all.
If concurrent INSERTs could lead to a different result (make different rows obsolete), you have to lock the whole table in EXCLUSIVE mode instead to avoid race conditions:
```
LOCK TABLE vendor_prices IN EXCLUSIVE MODE;
```
Do that only if it's necessary. It blocks all concurrent write access.
I am returning the number of rows deleted, but that's totally optional. You might as well return nothing and declare the function as RETURNS void.